I have a soft spot for chess variants. I think it's my hope that there's one of them that I will actually turn out to be good at, since I'm actually a very poor chess player.
Just a little while ago, I was telling a client about Kriegspiel, a variant in which you cannot see your opponent's pieces. Traditionally, you have a referee who sees the board behind a screen, though I suppose now you can do it with software. A player tries making a move, but then the referee tells them whether or not the move was legal. Unfortunately, I was telling my client about this as a metaphor for their software: their interface had you manipulating data without being able to see it very well, with lots of unpleasant, surprising error messages saying "you can't do that."
Kriegspiel is one of the more famous variants, but there are a number of popular others and there are seemingly endless obscure variations, so probably someone out there has already played the game I played in my dream.
The game started with an empty board and the normal set of pieces, in the custody of each player. Each turn, you could make a normal move or deploy a new piece onto any square. In the dream, this made the early phase of placing pieces seem more like Go than like chess.
I have no idea if this is actually a playable game. In the dream, there was also a rule that you couldn't place a piece to create a situation of check or checkmate. But I think no reasonable player would place their king until last anyway. In fact, it's a flaw of the game as I dreamed it that you could withhold your king, and not place it at all! Fortunately for my dream, I woke up before I got to the end of the placement process.
You could solve that problem by having to place the king as your first move, with the rule that you cannot put your opponent's king into check until you've placed all of your pieces. That's a kind of cool reversal of the normal rules for check, which appeals to my sense of whimsy. And the question of whether to place the king centrally or on the edge adds an interesting layer of playing style.
Alternatively, you could have some rule that compels completion of the placement process. Maybe you must place your king as your next move after you place your queen. This would lead to a player who's losing the advantage wanting to work their way out by summoning their queen, but then the opponent with the advantage would have the choice of countering with a queen with the knowledge of where the first player's queen went. I don't know if that would work. Maybe when one player places their king, the other player must place their king as the following move, which might produce some interesting play where a player who thinks they have an advantage can precipitate the question of kings.
However you solve the king placement problem, you get a game with two phases. It starts with a placement phase, which is more purely about territory, like Go, then switch to a king-hunting phase more like normal chess. Solving the problem of king placement is the key to making it a real game, I think. Though maybe there's something about the board, too; it might make sense to make it a touch bigger, say 10x10.